Plasma processing apparatus

ABSTRACT

An apparatus, which performs a plasma process on a target substrate by using plasma, includes first and second electrodes in a process chamber to oppose each other. An RF field, which turns a process gas into plasma by excitation, is formed between the first and second electrodes. An RF power supply, which supplies RF power, is connected to the first or second electrode through a matching circuit. The matching circuit automatically performs input impedance matching relative to the RF power. A variable impedance setting section is connected to a predetermined member, which is electrically coupled with the plasma, through an interconnection. The impedance setting section sets a backward-direction impedance against an RF component input to the predetermined member from the plasma. A controller supplies a control signal concerning a preset value of the backward-direction impedance to the impedance setting section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims the benefitof priority under 35 U.S.C. § 120 from, U.S. application Ser. No.10/615,915, filed Jul. 10, 2003; which claims the benefit of priorityunder 35 U.S.C. § 119 from Japanese Patent Applications Nos.2002-204928, filed Jul. 12, 2002 and 2003-60670, filed Mar. 6, 2003; andprior U.S. Provisional Patent Application Ser. No. 60/396,730, filedJul. 19, 2002, now abandoned. The entire contents of each of the aboveapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus which performs a plasmaprocess on a target substrate by using plasma in, e.g., a semiconductorprocessing system. The term “semiconductor process” used herein includesvarious kinds of processes which are performed to manufacture asemiconductor device or a structure having wiring layers, electrodes,and the like to be connected to a semiconductor device, on a targetsubstrate, such as a semiconductor wafer or an LCD substrate, by formingsemiconductor layers, insulating layers, and conductive layers inpredetermined patterns on the target substrate.

2. Description of the Related Art

In general, in the manufacture of a semiconductor device, variousprocesses, such as film formation, annealing, etching, oxidation anddiffusion, and the like, are performed. Most of these processes areperformed in a plasma processing apparatus using radio-frequency (RF)power.

For example, in a parallel plate plasma processing apparatus, asemiconductor wafer is placed on a lower electrode also serving as asusceptor. RF power is applied across the lower electrode and an upperelectrode opposing it, to generate plasma. Various processes, such asfilm formation and etching, are performed with the plasma.

To increase the yield of the products manufactured from a semiconductorwafer, the planar uniformity of the plasma process for the wafer must bemaintained high. In this case, the plasma process uniformity for thesemiconductor wafer largely depends on the state of the plasma generatedin the process chamber. Hence, conventionally, to optimize the plasmastate, the pressure or temperature in the process chamber during theprocess is adjusted. Also, the gas ratio of the various gases suppliedinto the process chamber is adjusted. Alternatively, the gap between theupper and lower electrodes is finely adjusted.

In the conventional apparatus, a structure that can adjust the gapbetween the upper and lower electrodes tends to be employed, becausethis structure is particularly effective in controlling the plasmastate. For example, an elevating mechanism for vertically moving thelower electrode is provided at the bottom of the process chamber, sothat the lower electrode can be moved vertically. The lower electrode isvertically moved when necessary by using the elevating mechanism, andthe gap between the lower and upper electrodes is adjusted.

In the plasma processing apparatus as described above in which theelectrode can be vertically moved, the plasma can be maintained in agood state regardless of the process conditions and the condition of theapparatus itself. However, for example, the lower electrode itself mustbe able to vertically move while maintaining the airtight state of theinterior of the apparatus. Also, the elevating mechanism and a motor forvertically moving the lower electrode must be provided. Consequently,not only the apparatus size becomes large, but also the cost increases.As the size of the apparatus itself becomes large, the space needed toinstall the apparatus, i.e., the footprint, also increases undesirably.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a plasmaprocessing apparatus in which the plasma state can be adjustedoptimally, so that the planar uniformity of the plasma process can bemaintained high with a simple structure.

It is a second object of the present invention to provide a plasmaprocessing apparatus in which the plasma state in the process chambercan be maintained stably with a simple structure.

It is a third object of the present invention to provide a calibrationmethod of performing calibration such that a machine difference(individual difference) concerning an impedance setting section used ina plasma processing apparatus is eliminated.

According to a first aspect of the invention, there is provided anapparatus which performs a plasma process on a target substrate by usingplasma, comprising:

an airtight process chamber which accommodates the target substrate;

a gas supply system which supplies a process gas into the processchamber;

an exhaust system which exhausts an interior of the process chamber andsets the interior of the process chamber to a vacuum state;

first and second electrodes arranged in the process chamber to opposeeach other, an RF field; which turns the process gas into plasma byexcitation, being formed between the first and second electrodes;

an RF power supply which is connected to the first or second electrodethrough a matching circuit and which supplies RF power, the matchingcircuit serving to automatically perform input impedance matchingrelative to the RF power;

an impedance setting section which is connected, through aninterconnection, to a predetermined member to be electrically coupledwith the plasma in the plasma process, and which sets abackward-direction impedance as an impedance against an RF componentinput from the plasma to the predetermined member, the impedance settingsection being capable of changing a value of the backward-directionimpedance; and

a controller which supplies a control signal concerning a preset valueof the backward-direction impedance to the impedance setting section.

The term “backward direction” is used in the specification because theRF component described above flows in a direction electrically oppositeto a direction in which a current flows from the RF power supply to thefirst or second electrode in the process chamber. Specifically, thedirection in which a current flows from the RF power supply to the firstor second electrode is defined as the forward direction, while thedirection opposite thereto is defined as the backward direction.

According to a second aspect of the invention, there is provided anapparatus which performs a plasma process on a target substrate by usingplasma, comprising:

an airtight process chamber which accommodates the target substrate;

a gas supply system which supplies a process gas into the processchamber;

an exhaust system which exhausts an interior of the process chamber andsets the interior of the process chamber to a vacuum state;

first and second electrodes arranged in the process chamber to opposeeach other, an RF field, which turns the process gas into plasma byexcitation, being formed between the first and second electrodes;

an RF power supply which is connected to the first or second electrodethrough a matching circuit and which supplies RF power, the matchingcircuit serving to automatically perform input impedance matchingrelative to the RF power;

an impedance setting section which is connected, through aninterconnection, to a predetermined member to be electrically coupledwith the plasma in the plasma process, and which sets abackward-direction impedance as an impedance against one of a pluralityof different higher harmonics relative to a fundamental frequency of theRF power input from the plasma to the predetermined member, theimpedance setting section being capable of changing a value of thebackward-direction impedance; and

a controller which supplies a control signal concerning a preset valueof the backward-direction impedance to the impedance setting section.

According to a third aspect of the invention, there is provided anapparatus which performs a plasma process on a target substrate by usingplasma, comprising:

an airtight process chamber which accommodates the target substrate;

a gas supply system which supplies a process gas into the processchamber;

an exhaust system which exhausts an interior of the process chamber andsets the interior of the process chamber to a vacuum state;

first and second electrodes arranged in the process chamber to opposeeach other, an RF field, which turns the process gas into plasma byexcitation, being formed between the first and second electrodes;

first and second interconnections which are respectively connected tothe first and second electrodes and which extend to an outside of theprocess chamber, the first and second interconnections forming part ofan AC circuit including electrical coupling between the first and secondelectrodes;

a first RF power supply which is arranged on the first interconnectionand which supplies first RF power;

a first matching circuit which is arranged on the first interconnectionbetween the first electrode and the first RF power supply and whichautomatically performs input impedance matching relative to the first RFpower;

an impedance setting section which is arranged on the secondintersection and which sets a backward-direction impedance as animpedance against an RF component input from the plasma to the secondelectrode, the impedance setting section being capable of changing avalue of the backward-direction impedance, and the RF componentincluding a component having a fundamental frequency of the first RFpower; and

a controller which supplies a control signal concerning a preset valueof the backward-direction impedance to the impedance setting section.

According to a fourth aspect of the invention, there is provided anapparatus which performs a plasma process on a target substrate by usingplasma, comprising:

an airtight process chamber which accommodates the target substrate;

a gas supply system which supplies a process gas into the processchamber;

an exhaust system which exhausts an interior of the process chamber andsets the interior of the process chamber to a vacuum state;

first and second electrodes arranged in the process chamber to opposeeach other, an RF field, which turns the process gas into plasma byexcitation, being formed between the first and second electrodes;

first and second interconnections which are respectively connected tothe first and second electrodes and which extend to an outside of theprocess chamber, the first and second interconnections forming part ofan AC circuit including electrical coupling between the first and secondelectrodes;

a first RF power supply which is arranged on the first interconnectionand which supplies first RF power;

a first matching circuit which is arranged on the first interconnectionbetween the first electrode and the first RF power supply and whichautomatically performs input impedance matching relative to the first RFpower;

an impedance setting section which is arranged on the first intersectionand which sets a backward-direction impedance as an impedance against anRF component input from the plasma to the first electrode, the impedancesetting section being capable of changing a value of thebackward-direction impedance, and the RF component including a harmonicof a fundamental frequency of the first RF power; and

a controller which supplies a control signal concerning a preset valueof the backward-direction impedance to the impedance setting section.

According to a fifth aspect of the invention, there is provided anapparatus which performs a plasma process on a target substrate by usingplasma, comprising:

an airtight process chamber which accommodates the target substrate;

a gas supply system which supplies a process gas into the processchamber;

an exhaust system which exhausts an interior of the process chamber andsets the interior of the process chamber to a vacuum state;

first and second electrodes arranged in the process chamber to opposeeach other, an RF field, which turns the process gas into plasma byexcitation, being formed between the first and second electrodes;

first and second interconnections which are respectively connected tothe first and second electrodes and which extend to an outside of theprocess chamber, the first and second interconnections forming part ofan AC circuit including electrical coupling between the first and secondelectrodes;

a first RF power supply which is arranged on the first interconnectionand which supplies first RF power;

a first matching circuit which is arranged on the first interconnectionbetween the first electrode and the first RF power supply and whichautomatically performs input impedance matching relative to the first RFpower;

an impedance setting section which is arranged on the first intersectionand which sets a backward-direction impedance as an impedance against anRF component input to the first electrode;

a second RF power supply which is arranged on the second interconnectionand which supplies second RF power, the second RF power supply beingcapable of changing a frequency of the second RF power;

a second matching circuit which is arranged on the secondinterconnection between the second electrode and the second RF powersupply and which automatically performs input impedance matchingrelative to the second RF power; and

a controller which supplies a control signal concerning a preset valueof a frequency of the second RF power to the second RF power supply.

According to a sixth aspect of the invention, there is provided acalibration method for the impedance setting section in the apparatusaccording to the first aspect, the method comprising steps of:

obtaining, by measurement, calibration data that compensates for adifference in setting the backward-direction impedance which isintrinsic to the impedance setting section; and adjusting the presetvalue with the calibration data and then adjusting thebackward-direction impedance.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a circuit diagram showing a matching circuit and impedancesetting section connected to a lower electrode in the apparatus shown inFIG. 1;

FIG. 3 is a graph showing the correlation between the adjustment value(dial value) and capacitance of the impedance setting section in theapparatus shown in FIG. 1;

FIG. 4 is a graph showing the correlation between the adjustment value(dial value) and reactance of the impedance setting section in theapparatus shown in FIG. 1;

FIG. 5 is a graph showing the correlation between the dial values ofprocesses A and B and a planar uniformity 3σ of the plasma process inthe apparatus shown in FIG. 1;

FIGS. 6A to 6C are graphs showing the distribution of the etching rateon a wafer with a diameter of 300 mm when the processes are performedwith a conventional apparatus and the apparatus shown in FIG. 1;

FIGS. 7A to 7G are circuit diagrams showing modifications of theimpedance setting section in the apparatus shown in FIG. 1;

FIG. 8 is a diagram showing plasma stability in the apparatus shown inFIG. 1 which is obtained when the combination of RF powers to be appliedto the upper and lower electrodes is changed;

FIG. 9 is a diagram showing the correlation between the dial value ofthe impedance setting section and the plasma stability in the apparatusshown in FIG. 1;

FIG. 10 is a diagram showing how a reactance measurement unit isattached in the apparatus shown in FIG. 1 when performing calibration;

FIGS. 11A to 11C are graphs schematically showing the correlationbetween the dial value and reactance, the correlation between the dialvalues before and after calibration, and the correlation between thedial value and reactance, respectively, of a plurality of (two) plasmaprocessing apparatuses each having the arrangement shown in FIG. 1;

FIG. 12 is a graph showing the correlation between the dial value andmatching position in the apparatus shown in FIG. 1;

FIG. 13 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to the fourth embodiment of the presentinvention, which uses an impedance setting section and avariable-frequency RF power supply;

FIG. 14 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to a fifth embodiment of the presentinvention, in which an impedance setting section is connected to anupper electrode;

FIG. 15 is a circuit diagram showing a matching circuit and impedancesetting section connected to the upper electrode in the apparatus shownin FIG. 14;

FIG. 16 is a graph showing a change in CD shift as a function of animpedance (13.56 MHz) in the apparatus shown in FIG. 14;

FIG. 17 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to a sixth embodiment of the presentinvention, in which an RF power supply is connected to only oneelectrode;

FIG. 18 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to a seventh embodiment of the presentinvention, in which a resonance impedance setting section is arranged;

FIG. 19 is a circuit diagram showing an example of the resonanceimpedance setting section in the apparatus shown in FIG. 18;

FIG. 20 is a graph showing the dependency of a bottom voltage Vpp as thevoltage value of the lower electrode on the capacitance of the variablecapacitor in the apparatus shown in FIG. 18;

FIGS. 21A to 21D are graphs showing the dependencies of respectiveharmonics including a fundamental wave on the capacitance of thevariable capacitor in the apparatus shown in FIG. 18;

FIG. 22 is a graph showing the dependency of the electron density inplasma on the capacitance of the variable capacitor in the apparatusshown in FIG. 18;

FIG. 23 is a graph showing the evaluation of the planar uniformity ofthe etching rate as a function of the capacitance of the variablecapacitor in the apparatus shown in FIG. 18;

FIGS. 24A to 24E are schematic views of a plasma processing apparatusaccording to the seventh embodiment of the present invention, to showhow a resonance impedance setting section is connected;

FIGS. 25A to 25C are circuit diagrams showing modifications of aresonance impedance setting section having a plurality of impedancechange units according to the seventh embodiment of the presentinvention;

FIG. 26 is a schematic view for explaining the respective connectionpoints of the circuit diagrams shown in FIGS. 25A to 25C;

FIGS. 27A to 27D are circuit diagrams showing examples of a high passfilter;

FIGS. 28A to 28D are circuit diagrams showing examples of a low passfilter;

FIG. 29 is a circuit diagram showing an example of a notch filter; and

FIG. 30 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus in which RF power supplies are respectivelyconnected to upper and lower electrodes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe accompanying drawings. In the following description, the constituentelements having substantially the same function and arrangement aredenoted by the same reference numeral, and a repetitive description willbe made only when necessary.

First Embodiment

FIG. 1 is a schematic diagram showing the arrangement of a plasmaprocessing apparatus according to the first embodiment of the presentinvention. As shown in FIG. 1, a plasma processing apparatus 2 has anairtight cylindrical process chamber 4. The process chamber 4 is madeof, e.g., aluminum, and can be vacuum-exhausted. The process chamber 4is grounded. An upper electrode 6 made of, e.g., aluminum, is attachedand fixed to the ceiling of the process chamber 4 through an insulatingmember 8. The upper electrode 6 forms a showerhead structure connectedto a gas supply unit GS. The showerhead structure 6 introduces variousgases, e.g., process gases, necessary for the process into the processchamber 4.

The upper electrode 6 is connected to an RF line 10. The RF line 10 isconnected to a first RF power supply 14 for plasma generation through afirst matching circuit 12 midway along it. The first RF power supply 14applies RF power of, e.g., 60 MHz, to the upper electrode 6. The firstmatching circuit 12 has an automatic matching function so that the inputimpedance becomes, e.g., 50Ω so as to prevent the RF power supplied fromthe first RF power supply 14 to the upper electrode 6 from beingreflected by the upper electrode 6.

The process chamber 4 has, in its bottom, exhaust ports 16 to beconnected to a vacuum exhaust unit ES including a vacuum pump or thelike. The vacuum exhaust unit ES exhausts the interior of the processchamber 4 and sets it in vacuum. A lower electrode 18 is arranged on asupport column standing vertically from the bottom of the processchamber 4 to oppose the upper electrode 6. The lower electrode 18 isinsulated. The lower electrode 18 is made of, e.g., aluminum, and alsofunctions as a susceptor. For example, a semiconductor wafer W is placedas a target substrate on the upper surface of the lower electrode 18.

A gate 20 which is opened/closed when loading/unloading the wafer W isdisposed on the sidewall of the process chamber 4. A rectifying plate 22for rectifying the exhaust gas is disposed to surround the lowerelectrode 18. The rectifying plate 22 is supported by the sidewall ofthe process chamber 4. With the rectifying plate 22, the atmospherewhich is being exhausted downward flows down uniformly from the lowerelectrode 18. A focus ring (not shown) made of, e.g., quartz or aceramic material, is disposed around the upper surface of the lowerelectrode 18. The focus ring focuses the plasma onto the surface of thewafer W.

The lower electrode 18 is connected to an RF line 24. The RF line 24 isconnected to a second RF power supply 28 for bias through a secondmatching circuit 26. The second RF power supply 28 for bias generates RFpower with a frequency of, e.g., 13.56 MHz. This frequency is lower thanthe frequency of the first RF power supply 14.

The RF lines 10 and 24 and the first and second RF power supplies 14 and28 form an AC circuit. This AC circuit includes electrical couplingbetween the upper and lower electrodes 6 and 18. The RF power appliedfrom the first RF power supply 14 to the upper electrode 6 is mainlyused for forming an RF field in a process space S between the lower andupper electrodes 18 and 6. The RF field turns the process gas intoplasma. The RF power applied from the second RF power supply 28 to thelower electrode 18 is mainly used for attracting ions in the plasma tothe surface of the wafer W. There is a case where the second RF powersupply 28 also generates plasma.

An impedance setting section 30 is arranged in the RF line 24 betweenthe second matching circuit 26 and lower electrode 18. The impedancesetting section 30 changes the impedance seen from the upper electrode 6side. In other words, the impedance setting section 30 sets abackward-direction impedance, which is an impedance against an RFcomponent input from the plasma to the lower electrode 18 due to the 60MHz power supplied from the first RF power supply 14 to the upperelectrode 6. The backward-direction impedance of the impedance settingsection 30 is adjusted by an impedance controller 32, e.g., amicrocomputer. Thus, the impedance setting section 30 is controlledappropriately.

More specifically, as shown in FIG. 2, the second matching circuit 26has a first fixed coil 34, first variable capacitor C1, and second fixedcoil 36. These components are connected in series on the RF line 24 fromthe lower electrode 18 (see FIG. 1) side toward the second RF powersupply 28 in this order.

A second variable capacitor C2 and fixed capacitor C3 are connected inparallel to each other between the two terminals of the second fixedcoil 36 and ground. The second matching circuit 26 has an automaticmatching function so that the input impedance becomes, e.g., 50Ω so asto prevent reflection of the RF power, supplied from the second RF powersupply 28 to the lower electrode 18, from returning into the second RFpower supply 28 (in the same manner as in the first matching circuit 12described above). At this time, the adjustment position (correspondingto the capacity) of the first variable capacitor C1 which changesautomatically can be checked with a position sensor 38. The current ofthe first RF power supply 14 supplied from the upper electrode 6 flowsto ground through the sidewall of the process chamber 4, the lowerelectrode 18, and the like. Conversely, the current of the second RFpower supply 28 supplied from the lower electrode 18 flows to groundthrough the sidewall of the process chamber 4, the upper electrode 6,and the like.

The impedance setting section 30 has a fixed coil 40 and variablecapacitor 42 connected in series between the RF line 24 and ground. Forexample, the fixed coil 40 has an inductance of substantially 200 nH.The impedance of the lower electrode 18 side seen from the upperelectrode 6 applied with 60 MHz power is set by changing the capacitanceof the variable capacitor 42. At this time, the capacitance value of thevariable capacitor 42 is automatically changed by an adjusting member 44connected to it. The dial adjustment value (to be referred to as dialvalue hereinafter) representing the impedance set value at this time isdisplayed by the adjusting member 44 or the like. At this time, thevalue of the impedance itself may also be displayed simultaneously. Animpedance is input as a dial value from the impedance controller 32 tothe adjusting member 44. The impedance is instructed based on a recipedefining the process conditions or the like for processing the wafer. Inplace of or together with the function of displaying the impedance setvalue or dial value, the adjusting member 44 may have a function oftransmitting (outputting) information to a host controller.

The inductance of the fixed coil 40 and the capacitance (including avariable range) of the variable capacitor 42 of the impedance settingsection 30 are set to provide such an impedance against the frequency of13.56 MHz of the second RF power supply 28, that is at least twicelarger than the load impedance formed by the process chamber 4 and theplasma generated in it. Consequently, even when the inductance of theimpedance of the impedance setting section 30 changes, it hardlyadversely affects the matching operation of the second matching circuit26. Also, this can prevent power loss of the RF power as the result ofthe presence of the impedance setting section and burn loss of theimpedance setting section accompanying it.

FIG. 3 shows an example of the correlation between a dial value DV ofthe adjusting member 44 and the capacitance of the variable capacitor42. When the dial value DV is 0 to 20, the capacitance can changesubstantially linearly within the range of about 5 pF to 130 pF. Theimpedance setting section 30 is set such that the larger the dial valueDV, the smaller the capacitance.

FIG. 4 shows the correlation between the dial value DV of the adjustingmember 44 and the reactance of the impedance setting section against 60MHz applied to the upper electrode 6. As is apparent from FIG. 4, thereactance can be controlled within the range of 30Ω to +600Ω by changingthe dial value DV within the range of 5 to 20.

The operation of this embodiment having the above arrangement will bedescribed.

As an example of the plasma process, a case will be described wherein agate electrode is formed by etching a polysilicon film formed on anunderlying layer formed of a silicon dioxide film. A process A with ahigh etching rate and an overetching process B with a low etching rateare performed continuously in one plasma processing apparatus. Of thetwo process steps, etching with a high anisotropy is performed in theprocess A in order to make the shape, and etching with a very highselectivity with respect to the underlying layer is performed in theprocess B.

The processes A and B use different conditions from each other, e.g.,the gas ratio of the supply amounts of a plurality of gases to besupplied, the supply power, and the process pressure. Accordingly, thestate of the plasma generated in the process space S changes. At thistime, the impedance setting section 30 is controlled to maintain theplanar uniformity of the plasma process.

The optimal impedance set values of the impedance setting section 30 forthe processes A and B are experimentally obtained in advance. Whenperforming the processes A and B, each impedance set value is input as adial value from a main controller CPU to the impedance setting section30 through the impedance controller 32. The main controller CPU controlsthe entire operation of the plasma processing apparatus 2. The impedancesetting section 30 automatically changes the capacitance of the variablecapacitor 42 to correspond to the dial value. Consequently, theimpedance is adjusted to an optimal value.

Examples of the process conditions for the processes A and B are asfollows: <Process A> Process gas (etching gas): HBr/O₂ = 400/1 sccmProcess pressure: 2.7 Pa (20 mTorr) Lower electrode temperature: 75° c.RF power: upper electrode/lower electrode = 200/100 W (watt) <Process B>Process gas (etching gas): HBr/O₂ = 1,000/4 sccm Process pressure: 20 Pa(150 mTorr) Lower electrode temperature: 75° C. RF power: upperelectrode/lower electrode = 650/200 W (watt)

An experiment performed for evaluating the uniformity controlcharacteristics for the processes A and B will be described. In thisexperiment, a step of etching a uniform polysilicon wafer (poly solidwafer) with no resist pattern for 25 sec was performed while changingthe dial value of the impedance setting section 30 little by little.FIG. 5 is a graph showing the correlation between the dial values DV ofthe processes A and B and a planar uniformity 3σ of the plasma process.Note that a represents standard deviation.

As is apparent from FIG. 5, the impedance of the impedance settingsection 30 was changed little by little while changing the dial value.Consequently, the planar uniformity changed largely, and a dial valuewith which the planar uniformity became minimum existed for each of theprocesses A and B. In this case, in the process A, the dial value withwhich the planar uniformity became minimum was approximately 11.5. Inthe process B, the dial value with which the planar uniformity becameminimum was approximately 15.8.

The processes A and B were performed with each of the conventionalapparatus and the apparatus of this embodiment. The obtained evaluationresult will be described.

FIGS. 6A to 6C are graphs showing the distribution of an etching rate ERon a wafer with a diameter of 300 mm when the processes are performedwith the conventional apparatus and the apparatus of this embodiment. Asthe conventional apparatus, one in which both the upper and lowerelectrodes were fixed and the impedance was adjusted for the process Bwas used.

FIG. 6A shows a result obtained by performing the process A with theconventional apparatus. A result obtained by performing the process Bwith the conventional apparatus is omitted. FIG. 6B shows a resultobtained by performing the process A with the apparatus of thisembodiment. FIG. 6C shows a result obtained by performing the process Awith the apparatus of this embodiment.

When the process B was performed with the conventional apparatus,although not shown, a good planar uniformity was obtained concerning theplasma process. When the process A was performed with the conventionalapparatus, as shown in FIG. 6A, the etching rate ER was low at the wafercenter, and increased toward the peripheral portion of the wafer. Theplanar uniformity 3σ of etching became worse to about 14.4%.

In contrast to this, with the apparatus of this embodiment, by referringto the result shown in FIG. 5 described above, the process A wasperformed with a dial value DV of 11.5, and the process B was performedwith a dial value DV of 15.8. As a result, as shown in FIG. 6B, with theprocess A, the planar uniformity 3σ of the plasma process was maintainedas very high as about 3.2%. As shown in FIG. 6C, with the process B, theplanar uniformity 3σ of the plasma process was maintained as fairly highas about 7.0%. Hence, with the apparatus of this embodiment, the planaruniformity of the plasma process was maintained high in both theprocesses A and B.

The impedance setting section 30 can be formed mainly of an inexpensive,very small electrical element. Hence, as compared to the conventionalplasma processing apparatus in which one of the upper and lowerelectrodes can vertically move, the structure is very simple, and theinstallation space can be greatly decreased. The employed dial values ofthe impedance setting section 30 are merely examples, and the optimalvalue changes in accordance with the process conditions or the like.

FIG. 2 shows a case wherein, as the impedance setting section 30, aseries connection circuit of the fixed coil 40 and variable capacitor 42is connected between the RF line 24 and ground. Alternatively, theimpedance setting section 30 may employ circuit configurations as shownin, e.g., FIGS. 7A to 7G. FIGS. 7A to 7G are circuit diagrams showingmodifications of the impedance setting section 30.

FIG. 7A shows a circuit in which a fixed coil 40 and variable capacitor42 are interchanged. FIG. 7B shows a circuit in which a variable coil 50capable of changing the inductance, and a fixed capacitor 52 areconnected in series. In place of the fixed capacitor 52, a variablecapacitor 42 may be provided. FIG. 7C shows a circuit in which a seriescircuit of a variable capacitor 42 and fixed coil 55 is connected inparallel to a fixed coil 40. With this arrangement, the series resonanceof the variable capacitor 42 and fixed coil 55 can minimize theimpedance. The parallel resonance of the variable capacitor 42, fixedcoil 40, and fixed coil 55 can maximize the impedance.

FIG. 7D shows a circuit in which a series circuit of a variable coil 50and fixed capacitor 54, and a fixed capacitor 52 are connected inparallel to each other. FIG. 7E shows a circuit in which a parallelconnection circuit of a fixed capacitor 52 and fixed coil 40, anotherfixed capacitor 54, and a variable coil 50 are sequentially connected inseries in this order. In this circuit, for example, the parallelresonance frequency obtained by the fixed coil 40 and fixed capacitor 52is matched with the frequency of the second RF power supply 28. Then,the impedance of the impedance setting section 30 against the RF powersupply 28 can be reliably increased to about 10 times or more.

In the circuit shown in FIG. 7F, switches 53 are respectively connectedin series to a plurality of capacitors 52. The switches 53 are turnedon/off in an arbitrary combination, so the capacitance is changedstepwise. In the circuit shown in FIG. 7G, switches 53 are respectivelyconnected in series to a plurality of inductors 40. The resultant seriescircuits are combined with a variable capacitor 42. When the switches 53are turned on/off in an arbitrary combination, the inductance is changedstepwise. Fine adjustment is performed with the variable capacitor, andrough adjustment is performed by switching the inductors. As a result, awide control range can be obtained while enabling fine control.

Where there are two target impedance values largely distant from eachother, rough adjustment is performed by switching fixed circuitelements. Then, fine adjustment is performed by continuously changingthe frequency. A change in impedance as the target can be realized withhigh precision.

Second Embodiment

In the first embodiment, a process mainly aimed at improving the planaruniformity of the plasma process is described. Sometimes a process inwhich the plasma stability must be maintained high may be performed.

Regarding the plasma stability, sometimes the plasma in the processspace S may or may not leak below the rectifying plate 22 (see FIG. 1)depending on the process conditions, e.g., the RF power to be applied,the process pressure, or the like. Between these cases, the impedanceobtained when seeing the plasma in the process chamber 4 from the firstor second matching circuit 12 or 26 (see FIG. 1) differs. Accordingly,the matching circuits 12 and 26 automatically change the impedances toperform input impedance matching, i.e., change the adjustment positions,as described above.

In this case, if the plasma stabilizes without or while leaking, noproblems occur. Assume, however, that the plasma is in the intermediatestate, i.e., in the critical state between leaking and non leaking. Inthis case, the adjustment position repeats changing often, so that thematching circuits can perform impedance matching. Therefore, plasmadischarge does not stabilize, and in the worst case, the plasma itselfis not generated.

In view of this, according to the second embodiment, to stabilize theplasma, an impedance setting section 30 identical to that used in thefirst embodiment is used. The arrangement of the entire plasmaprocessing apparatus is completely the same as that of the firstembodiment. As a plasma process, a case will be described wherein ananti reflection coating made of an organic substance and formed under aphotoresist film is to be etched.

The process conditions in this case are as follows: Process gas (etchinggas): CF₄/O₂ = 70/10 sccm Process pressure: 0.67 Pa (5 mTorr) Lowerelectrode temperature: 60° C.

An experiment on the plasma stability will be described. In thisexperiment, the plasma process was performed while changing thecombination of the RF powers to be applied to the upper and lowerelectrodes 6 and 18 in various manners. The obtained plasma stabilitywas visually checked. The RF power to the upper electrode was changedwithin the range of 100 W to 500 W. The RF power to the lower electrodewas changed within the range of 30 W to 105 W. The dial value of theimpedance setting section 30 was fixed at 15.2.

FIG. 8 shows the obtained evaluation result. FIG. 8 is a diagram showingplasma stability which is obtained when the combination of RF powers tobe applied to the upper and lower electrodes is changed. The criteriafor judgment of the plasma stability are as follows.

-   -   O: No flickering is visually observed above and below the        rectifying plate. No fluctuation or hunching occurs in        reflection of the voltage or RF power.    -   Δ: Flickering is visually observed below the rectifying plate.        No fluctuation or hunching occurs in reflection of the voltage        or RF power.    -   x: Flickering is visually observed above and below the        rectifying plate.        -   Reflection of the voltage or RF power fluctuates largely.        -   Hunching occurs even once.        -   Operation ceases midway due to reflection error of RF power.

As is apparent from FIG. 8, the state of the plasma largely changeddepending on the combination of the powers respectively applied to upperand lower electrodes 6 and 18. Particularly, when the powers applied tothe upper and lower electrodes 6 and 18 were 200 W and 45 W,respectively, the plasma state was x and was accordingly very unstable.

In view of this, while maintaining power application showing theunstable state, i.e., while applying RF powers of 200W and 45W to theupper and lower electrodes 6 and 18, respectively, the dial value of theimpedance setting section 30 was variously changed. A change in plasmastate at this time was visually observed.

FIG. 9 shows the obtained evaluation result. FIG. 9 is a diagram showingthe correlation between a dial value DV of the impedance setting sectionand the plasma stability. As is apparent from FIG. 9, as a region wherethe plasma was generated stably (portion O), two regions existed, i.e.,a region A with a dial value DV of 11.4 to 11.6 and a region B with adial value DV of 15.1 to 15.2.

The region A is where the plasma stabilizes while leaking. The region Bis where the plasma stabilizes without leaking (no leaking occurs).

In this manner, when determining the process conditions, a dial valuefor the process may be appropriately selected and defined in advance.Then, the plasma process can be performed while the plasma is generatedstably. For example, a plasma process is performed by using a recipeincorporating the dial value defined as described above. When the dialvalue is appropriately selected, a wide range can be set for the processconditions, and the process margin can be enlarged.

The types of the processes and the corresponding dial values are merelyexamples. Various appropriate dial values can be determined inaccordance with the process conditions.

Third Embodiment

Concerning the impedance setting section 30 or the like as describedabove, it is generally prepared as one of a large number devicesmanufactured with the same standard in accordance with the number ofaccepted orders for plasma processing apparatuses. In this case, a smalldifference in characteristics inevitably occurs in each impedancesetting section 30 due to manufacture variance or the like. Morespecifically, the same correlation between the dial value of theimpedance setting section 30 and an actual reactance at that time is notalways established between different impedance setting sections 30.Rather, this correlation often differs due to the machine difference(individual difference) of the impedance setting section. Assume that aplasma process is performed with a predetermined dial value. In thiscase, with some apparatus, the process may be performed with a highplanar uniformity. With another apparatus, even when the process isperformed with the same dial value, a high planar uniformity may not beobtained.

For this reason, to compensate for an intrinsic difference of eachimpedance setting section and of a matching circuit 26 connected to it,calibration is performed. In this case, the reactance of the impedancesetting section 30 is used as the parameter for calibration.

FIG. 10 is a diagram showing how a reactance measurement unit isattached when calibration is to be performed in a plasma processingapparatus. As shown in FIG. 10, a plasma processing apparatus 2 in thiscase has completely the same arrangement as that described previouslywith reference to FIG. 1.

First, to measure the reactance, a reactance measurement unit 56 isattached to an output terminal 30A (lower electrode 18 side) of theimpedance setting section 30. The correlation between the dial value andreactance is measured by using an instrument, such as an impedanceanalyzer or network analyzer. In this case, the reactance in a directionof an arrow 60 of FIG. 10, i.e., a reactance including the impedancesetting section 30 and a second matching circuit 26, is measured.Empirically, the machine difference tends to be small on a side wherethe capacitance of a variable capacitor 42 is small, and large on a sidewhere the capacitance of the variable capacitor 42 is large.

FIGS. 11A, 11B, and 11C are graphs schematically showing the correlationbetween dial values DV of a plurality of (two) plasma processingapparatuses and their reactances. FIG. 11A shows the correlation betweena pre calibration dial value Y and a reactance X. FIG. 11B shows thecorrelation between the pre calibration dial value Y and a postcalibration dial value Y′. FIG. 11C shows the correlation between thepost calibration dial value Y′ and the reactance X. As described above,FIG. 11A shows the correlation between the dial values and reactances ofthe two plasma processing apparatuses NO1 and NO2 that should have thesame characteristics. FIG. 11A also shows a reference correlation 62 asthe predetermined reference.

When performing calibration, the differences between the referencecorrelation 62 and the correlations of the plasma processing apparatusesNO1 and NO2 are obtained. A calibration function or calibration table(calibration data) prepared to eliminate these differences is stored inan adjusting member 44 (see FIG. 2). FIG. 11B shows dial values beforeand after this calibration. In actual process control, when a dial valuein the recipe is instructed by an impedance controller 32 (see FIG. 2),the variable capacitor 42 is controlled based on this calibrationfunction or calibration table.

The calibration function can be obtained by using, e.g., a two pointcalibration scheme. The dial value Y in the reference correlation 62when the reactance is X1 is defined as Y′1, and that when the reactanceis X2 is defined as Y′2. The pre calibration dial value with which thereactance of the apparatus NO1 is X1 is defined as Y11, and that withwhich the reactance is X2 is defined as Y12. When a function Y′=a1·Y+b1,which is the simplest as the calibration function, is employed, thefollowing simultaneous system of equations can be obtained for the twopoints:Y′1=a1·Y11+b1Y′2=a1·Y12+b1

Coefficients a1 and b1 of the calibration function for the apparatus NO1can be expressed by the following equations:a1=(Y′1−Y′2)/(Y11−Y12)b1=Y′1−(Y′1−Y′2)−Y11/(Y11−Y12)

Coefficients a2 and b2 of the calibration function for the apparatus NO2can be obtained with the same procedure. As shown in FIG. 11B, thecorrelations (calibration function) between the pre calibration dialvalues Y and post calibration dial values Y′ of the apparatuses NO1 andNO2 can be expressed as two straight lines having different gradientsand intercepts. The factor of the machine difference may include themachine difference of the inductance of the fixed coil and the machinedifference of the minimum capacitance of the variable capacitor. In acalibration curve, the former influences a gradient a, and the latterinfluences an intercept b.

FIG. 11C shows the correlations between the dial values Y′ aftercalibration and the reactances X. When the pre calibration dial value Yis plotted along the axis of abscissa (FIG. 11A), the three curves arelargely separate from each other. When the post-calibration dial valueY′ is plotted along the axis of abscissa, these three curves almostcoincide with each other. Therefore, either the apparatus NO1 or NO2 hasthe same reactance X against the same dial value Y′. If this calibrationfunction is obtained in advance for each plasma processing apparatus,with process conditions (recipe) including the same dial value, forexample, the same plasma state can always be formed in the respectiveapparatuses regardless of the machine difference.

(Calibration Including Process Chamber: 1)

In the above case, the reactance measurement unit 56 is connected to theoutput terminal 30A of the impedance setting section 30. Then, thereactance seen from the direction of the arrow 60 is measured. Adifference may sometimes occur from one apparatus to another in thereactance, depending on the apparatus arrangement and the componentarrangement (exchange of a component and the like). In this case, asshown in FIG. 10, the reactance measurement unit 56 is connected to thelower electrode 18. The impedance setting section 30 and its RF powersupply side are separated from the apparatus. The reactance (when thefrequency is 60 MHz) seen from the direction of an arrow 64 is measuredin the same manner as described above.

The calibration function and calibration tables are stored in theadjusting member 44 (see FIG. 2) in the same manner as described above.Both calibration seen from the direction of the arrow 60, which isdescribed previously, and calibration seen from the direction of thearrow 64 can be performed. Thus, one impedance setting section 30 can beused for the respective apparatuses. The impedance setting section 30need not be replaced for another one having the same standard, andcalibration need not be performed again.

(Calibration Including Process Chamber: 2)

In the above calibration, the reactance measurement unit 56 is connectedto the lower electrode 18, and a change in reactance is measured.Although this method has high precision, it does not actually generateplasma. Thus, the change does not reflect a difference in resonancedepending on the wafer state or the process conditions. Regarding this,alternatively, plasma may be generated actually. The correlation betweenthe dial value and the adjustment position detected by the positionsensor 38 of the second matching circuit 26 may be measured (see FIG.2). More specifically, a difference in reactance occurs depending on thearrangement of the apparatus, the arrangement of the components, thewafer state, the process conditions, and the like. Accordingly, thebehavior of the matching adjustment position with respect to the dialvalue also fluctuates.

FIG. 12 is a graph showing the correlation between the dial value DV anda matching position MP. FIG. 12 also shows a reference correlation 66 asthe reference for the matching position MP and dial value DV. Thereference correlation 66 includes points where the correlation betweenthe matching position MP and dial value DV changes largely, i.e., twoinflection points P1 and P2. Calibration is performed by referring toeither one of the inflection points P1 and P2, e.g., the inflectionpoint P1.

For example, assume that a correlation 68 between the matching positionand dial value of a plasma processing apparatus N03 differs from thereference correlation 66 by a dial value M. In this case, for example, acalibration table that cancels this value M is created in advance. Thecalibration table is stored in the adjusting member 44 (see FIG. 2) inadvance. Then, calibration is performed.

In the above calibration, a case is described wherein the correlationbetween the matching circuit and dial value is obtained. In place ofthis, a correlation between another one or a plurality of otherparameters and the dial value can be utilized. Other parameters includethe voltage amplitude of the RF power applied from the electrode sidewhere the impedance setting section is connected, the adjustment valueof the matching circuit of this electrode side, the voltage amplitude ofthe RF power applied from a counter electrode side, the adjustment valueof the matching circuit of this electrode side, and an output from aspectroscope for etching end point detection. Alternatively, theimpedance controller or the like may have the function of automaticallychanging the dial value to acquire data concerning changes in parametersdescribed above, and performing the calibration scheme as describedabove automatically.

Fourth Embodiment

In the embodiments described above, the impedance setting section 30that can change the impedance is provided. In place of the impedancesetting section 30, an impedance setting section 70 with a fixedimpedance may be provided, as shown in FIG. 13. In this case, theimpedance setting section 70 is connected to one electrode, e.g., thelower electrode 18, while a variable frequency RF power supply 72 whichcan change the frequency of the RF power is connected to the opposingelectrode, e.g., the upper electrode 6. The frequency of the RF powergenerated by the RF power supply 72 is adjusted by a controller 71. Thisadjustment is based on a recipe defining the process conditions or thelike with which the wafer is to be processed.

In the RF power supply 72, when a fundamental frequency fo is 60 MHz, anappropriate fluctuation width ±Δf is about ±5%. As the variablefrequency RF power supply 72, an RF power supply disclosed in, e.g.,Jpn. Pat. Appln. KOKAI Publication No. 5 114819, and Jpn. Pat. Appln.KOKAI Publication No. 9 55347 (corresponding to U.S. Pat. No.5,688,357), or the like can be used. If a wider fluctuation range isneeded, it can be realized by switching a plurality of fixed circuitelements.

More specifically, when control of the power supply frequency andvariable impedance elements are used in combination, a wide controlrange of the impedance can be obtained in a variable frequency rangethat cannot ordinarily be obtained.

In this manner, when the frequency of the RF power supply 72 isvariable, a frequency with which the planar uniformity of the plasmaprocess becomes optimal can be set in accordance with the processconditions.

Fifth Embodiment

FIG. 1 and the like show a case where the impedance setting section 30and the like are interposed in the RF line 24 connected to the lowerelectrode 18. Alternatively, as shown in the fifth embodiment of FIG.14, an impedance setting section 30 having the same arrangement can bearranged only in an RF line 10 connected to an upper electrode 6.Alternatively, an impedance setting section 30 can be arranged in eachof RF lines 10 and 24.

FIG. 15 is a circuit diagram mainly showing a matching circuit andimpedance setting section connected to the upper electrode. A firstmatching circuit 12 has the same arrangement as that obtained byomitting the fixed coil 34 from the matching circuit 26 shown in FIG. 2.Regarding an impedance setting section 30, it is formed as a seriescircuit of a fixed capacitor 52 and variable coil 50. The inductances ofthe respective coils and the capacitances of the capacitors aredetermined in accordance with the frequency of a corresponding RF power.This is different from the case shown in FIG. 2. Description on animpedance controller, an adjustment member, a matching adjustmentposition sensor, and the like is omitted.

An evaluation result of an experiment in which a plasma ashing processis performed with the apparatus arrangement as shown in FIGS. 14 and 15will be described.

In this ashing process, a target substrate formed in the followingmanner was used. Namely, a 100 nm thick TEOS SiO₂ film was formed on asilicon wafer. An 80 nm thick BARC (organic based anti reflectioncoating) photoresist was deposited on the upper surface of the SiO₂film, thus forming the target substrate. The photoresist had a 400 nmthick, 180 nm wide line pattern.

The etching conditions for the BARC and SiO₂ were as follows. <BARC>Process gas (etching gas): CH₄/CHF₃/O₂ = 157/52/11 sccm Processpressure: 0.93 Pa (7 mTorr) Lower electrode temperature: 75° C. RFpower: upper electrode/lower electrode = 100/500 watt Overetching: 10%<SiO₂> Process gas (etching gas): C₄F₈/Ar = 17/400 sccm Processpressure: 5.3 Pa (40 mTorr) Lower Electrode Temperature: 75° C. RFpower: upper electrode/lower electrode = 600/600 watt Overetching: 20%

FIG. 16 is a graph showing a change in CD (Critical Dimension) shift asa function of an impedance Z (13.56 MHz) obtained by this experiment.The CD shift represents a difference between the width of TEOS SiO₂before etching and resist ashing and the width of the same afterphotoresist etching. The impedance was changed by using the fixedcapacitor 52 with a capacity of 55 pF and the variable coil 50.

Referring to FIG. 16, Iso indicates an isolated pattern, and Nestindicates a line and space (1:1). As shown in FIG. 16, when theimpedance Z against 13.56 MHz was changed, the CD shift amount couldaccordingly be changed to a certain degree, e.g., by about 10 nm atmaximum when the impedance Z was within the range of 40Ω to 50Ω.

Sixth Embodiment

FIG. 1 and the like show an apparatus in which the RF power supplies 14and 28 are connected to the upper and lower electrodes 6 and 18,respectively. Alternatively, an impedance setting section 30 can beapplied to an apparatus in which an RF power supply is connected to onlyone electrode. In this case, an impedance setting section 30 isconnected to an electrode opposing an electrode to which the RF powersupply is connected. For example, in the structure shown in FIG. 17,this electrode is a lower electrode 18 opposing an upper electrode 6 towhich a first RF power supply 14 is connected.

Seventh Embodiment

In the embodiments described above, adjustment control is performed bychanging the impedance which is obtained when seeing from either oneelectrode the other electrode. Alternatively, an impedance seen fromplasma generated in the process chamber may be controlled. The plasmagenerates various higher harmonics in response to the fundamental waveof the RF power applied to the plasma. The plasma state changes inaccordance with how the harmonics are released from the process chamber.Hence, an impedance setting section, the impedance set value of whichcan be changed as described above, is connected to a predeterminedmember to be electrically coupled with the plasma. The impedance of theimpedance setting section is set such that it can resonate with at leastone of the higher harmonics.

FIG. 18 is a diagram showing the arrangement of a plasma processingapparatus according to the seventh embodiment of the present invention,to which a resonance impedance setting section is provided. FIG. 19 is acircuit diagram showing an example of the resonance impedance settingsection. A first RF power supply 14 and first matching circuit 12identical to those shown in FIG. 1 are omitted for facilitatingunderstanding of the present invention.

In the apparatus shown in FIG. 18, a resonance impedance setting section80 is arranged in place of the impedance setting section 30 of the RFline 24 shown in FIG. 1. The impedance set value of the impedancesetting section 80 is adjusted by a controller 81. This adjustment isbased on a recipe or the like defining the process conditions with whichthe wafer is to be processed. This is the same as with the impedancesetting section 30.

13.56 MHz RF power as the fundamental wave is applied from a second RFpower supply 28 across lower and upper electrodes 18 and 6. Thisgenerates plasma in a process space S. The plasma generates higherharmonics, e.g., second, third, fourth, fifth harmonics . . . , inresponse to the fundamental wave. The impedance setting section 80variably sets the impedance seen from the plasma such that it canresonate with at least one of the plurality of higher harmonics. Asdescribed above, the 13.56 MHz RF current as the fundamental wave flowsto the ground through the upper electrode 6, the sidewall of a processchamber 4, and the like.

As shown in FIG. 19, the impedance setting section 80 is formed of aseries circuit of a filter 82 and one impedance change unit 84.Furthermore, the impedance change unit 84 is formed of a series circuitof a variable capacitor 86 and fixed coil 88.

The fundamental wave of the second RF power supply 28, i.e., 13.56 MHzin this case, is applied to the lower electrode 18 to which the filter82 itself is connected. The filter 82 directly connected to an RF line24 cuts off the fundamental wave. This aims at preventing thefundamental wave from flowing into the process chamber 4. The filter 82selects and allows passage of a frequency higher than that of thefundamental wave. As the filter 82, a high pass filter is used.

The capacitance of the variable capacitor 86 of the impedance changeunit 84 is variable. In this embodiment, the capacitance of the variablecapacitor 86 can be controlled by adjusting the impedance seen from theplasma. Then, resonance can be selected from a range of near a secondharmonic to near a fourth harmonic with respect to the fundamental wave.When a plasma process such as actual etching is to be performed, thevariable capacitor 86 of the impedance change unit 84 is variablyadjusted. This is to control such that the impedance seen from theplasma can selectively resonate with the second, third, or fourthharmonic. Then, the planar uniformity of the plasma process for a waferW can be maintained high. Also, the plasma state in the process chamber4 can be maintained stably.

The capacity of the variable capacitor 86 is changed in various manners.The fluctuation states of the voltages of the harmonics including thefundamental wave, the electron density at this time in the plasma, andthe etching planar uniformity are evaluated. The evaluation result willbe described. FIG. 20 is a graph showing the dependency of a bottomvoltage Vpp (see FIG. 18) as the voltage value of the lower electrode 18on the capacitance (dial value DV) of the variable capacitor. FIGS. 21Ato 21D are graphs showing the dependencies of the bottom voltages Vpp ofthe respective harmonics including the fundamental wave on thecapacitance (dial value DV) of the variable capacitor. FIG. 22 is agraph showing the dependency of an electron density ED in the plasma onthe capacitance (dial value DV) of the variable capacitor. FIG. 23 is agraph showing the evaluation of the planar uniformity of an etching rateER as a function of the capacitance (dial value DV) of the variablecapacitor. In FIG. 22, the dial value DV of the variable capacitor 86 isexpressed as 0 to 11. This corresponds to a capacitance change of, e.g.,250 pF to 30 pF.

As is apparent from FIG. 20, at points A1, A2, and A3 where the dialvalue DV was “0”, “7.5”, and “9.9”, the bottom voltage Vpp leapedlargely, and resonance occurred at these points A1 to A3. A change involtage against the respective harmonics including the fundamental wavewas measured. Although second, third, and fourth harmonics are indicatedas examples, a further higher harmonic may also be considered.

FIG. 21A shows a change in bottom voltage Vpp against the fundamentalwave (13.56 MHz). The voltage temporarily decreases sharply, even if alittle, at the points A1, A2, and A3. FIG. 21B shows a change in bottomvoltage Vpp against a second harmonic (27.12 MHz). The voltage increasessharply at the point A1, and resonance with the second harmonic occurswhen the dial value DV is “0”. FIG. 21C shows a change in bottom voltageVpp against a third harmonic (40.68 MHz). The voltage increases sharplyat the point A2, and resonance with the second harmonic occurs when thedial value DV is “7.5”. FIG. 21D shows a change in bottom voltage Vppagainst a fourth harmonic (54.24 MHz). The voltage increases sharply atthe point A3, and resonance with the second harmonic occurs when thedial value DV is “9.9”.

A probe for measuring the electron density was inserted in the plasma,and the electron density ED was measured. As a result, as shown in FIG.22, the electron density ED decreases temporarily at the points A1, A2,and A3 (dial: 0, 7.5, and 9.9). It was confirmed that the plasma statewas controlled at these points.

On the basis of the above evaluation result, the silicon oxide film ofthe wafer was etched with various different dial values DV. The obtainedetching rate ER will be described with reference to FIG. 23. A waferhaving a diameter of 200 mm was used. The process conditions were asfollows. As the etching gas, CF4 was used. The flow rate of the etchinggas was 80 sccm. The process pressure was 150 mTorr (20 Pa).

FIG. 23 shows the points A1 to A3 and points B1 to B4 corresponding tothe respective dial values. The dial value was set at the points B1 toB4 that were off the resonance point, and etching was performed. As isapparent from FIG. 23, with any of these dial values, the etching ratewas high at the wafer center and low at the peripheral portion. Theplanar uniformity of the etching rate was poor.

When, the dial value, however, was set at the respective resonancepoints A1 to A3, the increase in etching rate at the wafer center wassuppressed, so the overall etching rate became substantially flat. Theplanar uniformity of the etching rate was largely improved. In thiscase, as the harmonic wave becomes fourth, third, or second harmonic,the etching rate gradually decreased in this order. Thus, to maintain ahigh etching rate, it is preferable to so adjust the impedance as toresonate particularly with the fourth harmonic. When the dial value isset at the point A1, although the planar uniformity can be improved, theetching rate itself becomes excessively low.

FIG. 19 shows an example in which a series circuit of the variablecapacitor 86 and fixed coil 88 is used as the impedance change unit 84.The impedance change unit 84 is not limited to this, but can be anycircuit as long as it can change the impedance. For example, all thecircuit configurations as shown in FIGS. 7A to 7G can be used. In thiscase, as described above, an impedance range that can be changed suchthat the impedance can resonate with a harmonic as opposed to thefundamental wave is set. As shown in FIGS. 7F and 7G, when the impedanceis switched by the switches 53, the inductance of the fixed coil 40 andthe capacitance of the fixed capacitor 52 are set at such values thatthe impedance can resonate with a specific higher harmonic as thetarget.

FIG. 18 shows a case where the impedance setting section 80 is providedat the RF line 24 of the second RF power supply 28. The impedancesetting section 80 is not limited to this, but can be provided at anyportion where the RF current flows (in other words, any portionelectrically coupled with the plasma). FIGS. 24A to 24E are schematicviews showing portions where a resonance impedance setting section canbe connected. In FIGS. 24A to 24E, the plasma processing apparatus isschematically described, and how the resonance impedance setting sectionis connected is shown.

FIG. 24A shows a case where the impedance setting section 80 isconnected to the lower electrode 18 by using a line different from theRF line 10. FIG. 24B shows a case where the impedance setting section 80is connected to a focus ring 90. FIG. 24C shows a case where theimpedance setting section 80 is connected to the rectifying plate 22.FIG. 24D shows a case where the impedance setting section 80 isconnected to the wall (including the sidewall and bottom wall) of theprocess chamber 4. FIG. 24E shows a case where the impedance settingsection 80 is connected to the upper electrode 6. In the case shown inFIG. 24D, the process chamber 4 is not directly grounded regarding thehigher harmonics as the target, but is grounded through the impedancesetting section 80. All the connection states shown in FIGS. 24A to 24Ecan exhibit the same operation and effect as those described withreference to FIG. 18.

The resonance impedance setting section 80 can cope with resonance withthe second to fourth harmonics by means of the impedance change unit 84formed of one variable capacitor 86 and one fixed coil 88.Alternatively, a plurality of (three in this case) impedance changeunits may be provided so that the respective harmonics can be impedancecontrolled independently of each other. FIGS. 25A to 25C are circuitdiagrams showing modifications of the resonance impedance settingsection having a plurality of impedance change units. FIG. 26 is aschematic view for explaining the respective connection points of thecircuit diagrams shown in FIGS. 25A to 25C.

Symbols pa, pb, and pc indicating the three connection points of theimpedance setting section 80 shown in FIG. 26 are indicated at thecorresponding portions of FIGS. 24A to 24E, FIGS. 25A to 25C, and FIG.30. The connection point pc of FIG. 26 is open or connected to amatching point when the connection point pa is connected to an electrode(see FIG. 30). The connection point pc of FIG. 26 is open when theconnection point pa is connected to a member other than an electrode(see FIGS. 24A to 24E).

In the case shown in FIG. 25A, three bandpass filters 82A, 82B, and 82Cfor passing different harmonics are connected to the RF line 24 to beparallel to each other, to form the filter 82. In this case, the first,second, and third bandpass filters 82A, 82B, and 82C pass frequencybands respectively having the second, third, and fourth harmonics as thecentral frequencies. The bandpass filters 82A, 82B, and 82C do not passthe fundamental wave (13.56 MHz). Variable capacitors 86A, 86B, and 86Cand fixed coils 88A, 88B, and 88C are respectively, separately connectedin series to the bandpass filters 82A, 82B, and 82C. Three impedancechange units 84A, 84B, and 84C are thus formed. The impedance changeunits 84A, 84B, and 84C are separately connected in series to thebandpass filters 82A, 82B, and 82C, respectively.

According to this arrangement, the impedance can selectively resonatewith one of the three different higher harmonics. The impedance can alsoresonate with two or three arbitrary harmonics simultaneously.Therefore, the characteristics of the respective harmonics about theplasma process can be combined in a complex manner.

In the case shown in FIG. 25B, first, second, and third high passfilters 92A, 92B, and 92C are connected in series in this order to forma filter 82. The first high pass filter 92A passes any frequency equalto or higher than that of the second harmonic. The second high passfilter 92B passes any frequency equal to or higher than that of thethird harmonic. The third high pass filter 92C passes any frequencyequal to or higher than that of the fourth harmonic. An impedance changeunit 84A for the second harmonic is connected between the first andsecond high pass filters 92A and 92B. The impedance change unit 84A hasthe same arrangement as that shown in FIG. 25A. An impedance change unit84B for the third harmonic is connected between the second and thirdhigh pass filters 92B and 92C. An impedance change unit 84C for thefourth harmonic is connected downstream of the third high pass filter92C. In this case as well, the same operation and effect as thosedescribed with reference to FIG. 25A can be exhibited.

The circuit configuration shown in FIG. 25C is used in a circuitconfiguration as shown in FIG. 30 to be described later. Accordingly,this circuit configuration is employed on the premise that thefundamental wave flows through it. Hence, this circuit configuration isnot used if it is to be connected to the lower electrode 18 (see FIG.24A), focus ring 90 (see FIG. 24B), or rectifying plate 22 (see FIG.24C). Rather, this circuit configuration is used if it is to beconnected to the process chamber 4 (see FIG. 24D) or upper electrode 6(see FIG. 24E). This limitation does not apply when the circuitconfiguration is as shown in FIGS. 25A and 25B. As shown in FIG. 25C, afilter 82 is formed by connecting in series first, second, and third lowpass filters 94A, 94B, and 94C in this order. The first low pass filter94A passes any frequency equal to or lower than that of the fourthharmonic. The second low pass filter 94B passes any frequency equal toor lower than that of the third harmonic. The third low pass filter 94Cpasses any frequency equal to or lower than that of the second harmonic.

An impedance change unit 84C for the fourth harmonic is connectedbetween the first and second low pass filters 94A and 94B. The impedancechange unit 84C has the same arrangement as that shown in FIG. 25A. Animpedance change unit 84B for the third harmonic is connected betweenthe second and third low pass filters 94B and 94C. An impedance changeunit 84A for the second harmonic is connected downstream of the thirdlow pass filter 94C. In this case as well, the same operation and effectas those described with reference to FIG. 25A can be exhibited.

The high pass filters described in this embodiment can be formed asshown in, e.g., FIGS. 27A to 27D. FIG. 27A shows an arrangement formedof a fixed capacitor C1 and fixed resistor R1. The fixed capacitor C1 isconnected in series to the circuit. The fixed resistor R1 is connectedin parallel to the circuit. FIG. 27B shows an arrangement formed of afixed capacitor C1 and fixed coil L1. The fixed capacitor C1 isconnected in series to the circuit. The fixed coil L1 is connected inparallel to the circuit. FIG. 27C shows an arrangement formed of a fixedcapacitor C1 and a series circuit. The fixed capacitor C1 is connectedin series to the circuit. The series circuit is formed of a fixed coilL1 and fixed capacitor C2, and connected in parallel to the circuit.FIG. 27D shows an arrangement formed of a parallel circuit and a fixedcoil L2. The parallel circuit is formed of a fixed capacitor C1 andfixed coil L1, and connected in series to the circuit. The fixed coil L2is connected in parallel to the circuit.

The low pass filters described in this embodiment can be formed as shownin, e.g., FIGS. 28A to 28D. FIG. 28A shows an arrangement formed of afixed resistor R1 and fixed capacitor C1. The fixed resistor R1 isconnected in series to the circuit. The fixed capacitor C1 is connectedin parallel to the circuit. FIG. 28B shows an arrangement formed of afixed coil L1 and fixed capacitor C1. The fixed coil L1 is connected inseries to the circuit. The fixed capacitor C1 is connected in parallelto the circuit. FIG. 28C shows an arrangement formed of a fixed coil L1and a series circuit. The fixed coil L1 is connected in series to thecircuit. The series circuit is formed of a fixed capacitor C1 and fixedcoil L2, and connected in parallel to the circuit. FIG. 28D shows anarrangement formed of a parallel circuit and a fixed capacitor C2. Theparallel circuit is formed of a fixed coil L1 and fixed capacitor C1,and connected in series to the circuit. The fixed capacitor C2 isconnected in parallel to the circuit.

FIG. 29 is a circuit diagram showing an example of a notch filter. Inplace of the bandpass filters 82A to 82C described above, a notch filterof this type may be used. In the notch filter, a notch that does notpass only a specific frequency band is connected in series. Thus, thenotch filter passes a desired frequency band. For example, the parallelcircuit of a first fixed coil L1 and first fixed capacitor C1 cuts thefrequency band of the fundamental wave. The parallel circuit of a secondfixed coil L2 and second fixed capacitor C2 cuts the frequency band ofthe second harmonic. The parallel circuit of a third fixed coil L3 andthird fixed capacitor C3 cuts the frequency band of the third harmonic.When these parallel circuits are connected in series, the notch filtercan pass the frequency band of the fourth harmonic (more particularly,the notch filter passes any frequency band equal to or higher than thatof the fourth harmonic). Accordingly, if the inductances of therespective fixed coils and the capacitances of the respective fixedcoils are appropriately set, the notch filter can cut any unwantedfrequency band and passes any desired frequency band.

In the seventh embodiment, the RF power supply 28 is connected to thelower electrode 18. If an RF power supply is connected to only the upperelectrode 6, the arrangement is merely reversed upside down, and thesame effect as that described above can be obtained. In this case, thelower electrode 18 is set such that the RF current applied to the upperelectrode 6 flows through it.

The seventh embodiment can also be applied to a case where RF powersupplies 14 and 28 are respectively connected to upper and lowerelectrodes 6 and 18 (this is the same as in the case shown in FIG. 1),as shown in FIG. 30. In FIG. 30, a resonance impedance setting section80 which can change the impedance set value is provided at an RF line 24for the lower electrode 18. A resonance impedance setting section 98which can change the impedance set value is also provided at an RF line10 for the upper electrode 6. In this case, in the arrangement of theimpedance setting section 98 of the upper electrode 6 side, thefundamental frequency is changed from 13.56 MHz to 60 MHz of the firstRF power supply 14. Except for this, the arrangement previouslydescribed concerning the impedance setting section 80 of the lowerelectrode 18 side can entirely be applied to the arrangement of theimpedance setting section 98. Alternatively, either one of the twoimpedance setting sections 80 and 98 may be employed.

In the seventh embodiment, each impedance change unit is set to realizea complete resonance state with a higher harmonic, or to realize a statelargely off the resonance state from the higher harmonic. Alternatively,in the seventh embodiment, the plasma state may be controlled by settingan incomplete resonance state, e.g., a resonance state of about 50%.Also, the degree of the resonance state may be controlled to linearlychange within the range of 0% to 100%.

The frequencies of the RF power supplies employed in the first toseventh embodiments are merely examples. For example, 800 kHz, 2 MHz, 27MHz, 100 MHz, and the like can be used instead. Two or more of RF powersupplies of different frequencies may be connected to one electrode. Inthis case, for example, a combination of them, such as 40 MHz and 3.2MHz, 100 MHz and 3.2 MHz, or 40 MHz and 13.56 MHz may be used.

Furthermore, the respective embodiments can be employed when a targetsubstrate other than a semiconductor wafer, e.g., a glass substrate, LCDsubstrate, or the like is to be processed.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An apparatus which performs a plasma process on a target substrate byuse of plasma, the apparatus comprising: an airtight process chamberwhich accommodates the target substrate; a gas supply system whichsupplies a process gas into the process chamber; an exhaust system whichexhausts an interior of the process chamber and sets the interior of theprocess chamber to a vacuum state; first and second electrodes arrangedin the process chamber to oppose each other, an RF field, which turnsthe process gas into plasma by excitation, being formed between thefirst and second electrodes; a first RF power supply connected to thefirst electrode through a first interconnection and configured to supplyfirst RF power; a first matching circuit arranged between the firstelectrode and the first RF power supply on the first interconnection andconfigured to automatically perform input impedance matching relative tothe first RF power; an impedance setting section connected to the secondelectrode through a second interconnection and configured to set animpedance of the second electrode side seen from the first electrodeagainst a fundamental frequency of the first RF power, the impedancesetting section including an impedance change unit connected to thesecond interconnection and configured to change the impedance; and acontroller which supplies a control signal for controlling the impedancesetting section to control a characteristic of a plasma processperformed in the process chamber.
 2. The apparatus according to claim 1,further comprising: a second RF power supply connected to the secondelectrode through the second interconnection and configured to supplysecond RF power; and a second matching circuit arranged between theimpedance setting section and the second RF power supply on the secondinterconnection and configured to automatically perform input impedancematching relative to the second RF power.
 3. The apparatus according toclaim 2, wherein the first RF power has a frequency higher than that ofthe second RF power.
 4. The apparatus according to claim 2, wherein thefirst RF power has a frequency lower than that of the second RF power.5. The apparatus according to claim 1, wherein the controller furthercomprises a storage which stores data concerning a correlation betweenfirst and second processes having different conditions and first andsecond preset values of the impedance corresponding to the first andsecond processes, respectively, and the controller supplies to theimpedance setting section a control signal which changes the impedancefrom the first preset value to the second preset value in accordancewith the data when a process to be performed in the process chamberchanges from the first process to the second process.
 6. The apparatusaccording to claim 1, wherein the impedance change unit comprises one orboth of an arrangement which continuously changes the impedance with acontinuous variable element, and an arrangement which changes theimpedance stepwise by switching a plurality of fixed elements.
 7. Theapparatus according to claim 1, wherein the impedance setting sectioncomprises a function which displays a preset value.
 8. The apparatusaccording to claim 1, wherein the controller or the impedance settingsection corrects a preset value with calibration data that compensatesfor a difference intrinsic to the impedance setting section, and thenadjusts the impedance.
 9. The apparatus according to claim 2, wherein avalue of an impedance formed by the impedance setting section againstthe second RF power is not less than twice a value of an RF loadimpedance formed by the process chamber and the plasma against thesecond RF power.
 10. The apparatus according to claim 1, wherein thesecond interconnection is grounded through the impedance settingsection.
 11. An etching apparatus for etching a target substrate by useof plasma, the apparatus comprising: an airtight process chamber whichaccommodates the target substrate; a gas supply system which supplies aetching gas into the process chamber; an exhaust system which exhaustsan interior of the process chamber and sets the interior of the processchamber to a vacuum state; first and second electrodes arranged in theprocess chamber to oppose each other, an RF field, which turns theetching gas into plasma by excitation, being formed between the firstand second electrodes; a first RF power supply connected to the firstelectrode through a first interconnection and configured to supply firstRF power; a first matching circuit arranged between the first electrodeand the first RF power supply on the first interconnection andconfigured to automatically perform input impedance matching relative tothe first RF power; an impedance setting section connected to the secondelectrode through a second interconnection and configured to set animpedance of the second electrode side seen from the first electrodeagainst a fundamental frequency of the first RF power, the impedancesetting section including an impedance change unit connected to thesecond interconnection and configured to change the impedance; and acontroller which supplies a control signal for controlling the impedancesetting section, the controller being configured to apply differentpreset values of the impedance to first and second etching processesperformed within the process chamber and required to have differentetching rates, to maintain planar uniformities of the first and secondetching processes better than a predetermined level.
 12. The apparatusaccording to claim 11, wherein the first and second electrodes are upperand lower electrodes, respectively, and the second electrode isconfigured to support the target substrate thereon.
 13. The apparatusaccording to claim 12, further comprising: a second RF power supplyconnected to the second electrode through the second interconnection andconfigured to supply second RF power, and a second matching circuitarranged between the impedance setting section and the second RF powersupply on the second interconnection and configured to automaticallyperform input impedance matching relative to the second RF power. 14.The apparatus according to claim 13, wherein the first RF power has afrequency higher than that of the second RF power.
 15. The apparatusaccording to claim 11, wherein the controller further comprises astorage which stores data concerning a correlation between the first andsecond etching processes and first and second preset values of theimpedance corresponding to the first and second etching processes,respectively, and the controller supplies to the impedance settingsection a control signal which changes the impedance from the firstpreset value to the second preset value in accordance with the data whena process to be performed in the process chamber changes from the firstetching process to the second etching process.
 16. The apparatusaccording to claim 11, wherein the impedance change unit comprises oneor both of an arrangement which continuously changes the impedance witha continuous variable element, and an arrangement which changes theimpedance stepwise by switching a plurality of fixed elements.
 17. Theapparatus according to claim 11, wherein the impedance setting sectioncomprises a function which displays a preset value.
 18. The apparatusaccording to claim 11, wherein the controller or the impedance settingsection corrects a preset value with calibration data that compensatesfor a difference intrinsic to the impedance setting section, and thenadjusts the impedance.
 19. The apparatus according to claim 12, whereina value of an impedance formed by the impedance setting section againstthe second RF power is not less than twice a value of an RF loadimpedance formed by the process chamber and the plasma against thesecond RF power.
 20. The apparatus according to claim 15, wherein thecontroller is configured to sequentially perform main etching and overetching as the first and second etching processes, respectively, and theover etching is performed with an etching rate lower than that of themain etching.